Well let's see
Gas-3,6,9,12,15,18,21,24,27
Wash-8,16,24,
So on the 24th day.
I disagree with the student because the slope is the rate of change. The slope would be 10 because the rocks are increasing by 10 every month.
Using slope-intercept form, y = mx + b where m = slope and b = y-intercept:
We know our slope is -6. This can be interpreted as -6/1, which rise-over-run-wise, means that when y changes by 6, x changes inversely by 1.
To find that y-intercept, though, we need to find the value of y when x = 0.
Use our point (-9, -3) to find this...
We want to add 9 to x so that it becomes 0.
According to our slope, this means subtracting 54 from y.
Our y-intercept is at (0, -57), with -57 being the value of b we put in our equation.

You could also just use point-slope form:
y - y¹ = m(x - x¹)
y - (-3) = -6(x - (-9))
y + 3 = -6(x + 9)
And convert to slope-intercept if you want:
y + 3 = -6x - 54
y = -6x - 57
6r - 3t = 6 /*8
6r + 8t = - 16 /*3
48r - 24t = 48
48r + 24t = - 48
---------------------- +
96r = 0
r = 0
t = ( -16 + 6r)/8 = - 16/8 = - 2
Answer:
y=-6x-24
Step-by-step explanation:
just expand
y+6=-6x-18
then move the six from the left to the right
y=-6x-24